Method for calibrating the frequency of an rf oscillator in a mobile part of a mobile communications device

ABSTRACT

A method is provided for calibrating the frequency of an RF oscillator in a mobile part of a mobile communications device when the mobile part is first switched on, wherein the method includes the steps of: sampling a modulated carrier that is transmitted by a base station and has a prescribed frequency accuracy, such that scanning of the modulated carrier is performed via a frequency search algorithm with the modulated carrier being demodulated, a signal thus demodulated being compared in an evaluation and synchronization program via a correlation with a sequence known in advance, and a correlation peak in an output of the correlator being determined; determining a frequency error between the frequency of the RF oscillator in the mobile part of the mobile communications device and a known channel frequency of the base station by comparing the determined correlation peak of the output of the correlator with a channel raster, known in advance to the mobile communications device, of the base station; synchronizing the mobile communications device with the modulated carrier; and adopting a frequency, corrected by the frequency error, for the RF oscillator as reference frequency for further calibration operations.

[0001] The invention relates to a method for calibrating the frequency of an RF oscillator in a mobile part of a mobile communications device when the mobile part of the mobile communications device is first switched on, having the following steps: a) sampling a modulated carrier that is transmitted by a base station and has a prescribed frequency accuracy, b) determining the frequency error between the frequency of the RF oscillator in the mobile part of the mobile communications device and a known channel frequency of the base station, synchronization of the mobile communications device with the modulated carrier.

[0002] Such a method is disclosed in EP 0 735 675 A2, the determination of the sequence error being performed successively by virtue of the fact that the search range for the frequency of the base station is expanded until the frequency of the base station is found. The channel frequency of the base station is certainly fundamental, but not known on the part of the mobile communications device.

[0003] Depending on the transmission method, the field of mobile communication is generally covered by a certain standard that requires a prescribed frequency accuracy for a base station of a communications device during transmission and prescribes a frequency accuracy for the case of reception in the mobile receiver part via further system properties.

[0004] This standard has been ensured to date by calibrating the RF oscillator during fabrication of the mobile part of a mobile communications device. However, calibration in fabrication is very expensive and requires a long production time.

[0005] It has also already been proposed to make use, for the purpose of correcting the temperature dependence of an RF oscillator in a mobile part of a mobile communications device, of a temperature sensor that compensates the temperature-dependent error of the RF oscillator in conjunction with an appropriate algorithm. This method is also very expensive.

[0006] It is therefore the object of the present invention to propose a method by means of which the production costs for a mobile communications device are lowered and, at the same time, a reliable calibration of the frequency of the RF oscillator of the mobile part of the mobile communications device is ensured.

[0007] The object is achieved by a method having the features of claim 1. Developments of the invention are the subject matter of the subclaims.

[0008] The invention is based on the fundamental idea that the calibration of the RF oscillator can be performed by the customer when a mobile receiver is switched on for the first time. Use is made for this purpose of the accuracy of the transmit frequency of a base station, which accuracy is specified with sufficiently high accuracy and by standardization bodies. The transmitted carrier, with information modulated thereon, which is transmitted by the base station, is scanned in the mobile receive section. If the transmitted carrier, whose frequency is known a priori with high accuracy through the channel raster, is received, with the information modulated on, with a satisfactory frequency accuracy, the frequency error can be determined and the mobile part of the mobile communications device can be synchronized with this carrier. The frequency corrected by the frequency error is then used as reference frequency. Instead of the initial calibration during the production process of the mobile part, the required settings are undertaken during the first startup by means of the frequency search algorithm. The calibration is therefore performed fully automatically.

[0009] It is advantageous to carry out the method according to the invention repeatedly, in particular with each xth callup, starting in each case from the reference frequency last determined. It is then possible to assume a smaller frequency deviation of the RF oscillator in these later synchronization operations. As a rule, the method according to the invention is not carried out again immediately, since it may be assumed that the RF oscillator has been effectively calibrated by the previous initial autocalibration. The dummy x is a function, inter alia, of temperature fluctuations in the surroundings of the oscillator, and of its age. Consequently, the dummy x can certainly be 1, but is not constrained to be so. The future synchronization is thereby accelerated. Moreover, the accuracy is increased by averaging a plurality of measurements. In addition, the speed of the synchronization is increased by virtue of the fact that only a relatively small frequency band has to be scanned. The power consumption in the mobile part is reduced by a quicker synchronization, and this leads to a longer period of use. It is thereby possible, in addition, to recalibrate aging effects and temperature deviations of the components that are required for frequency generation. This can take place, for example, with each xth callup, and consequently does not cause any additional outlay. The recalibration also substantially lengthens the product service life without additional outlay.

[0010] It is also advantageous when use is made during the first synchronization of a transmitted signal of an arbitrary base station of sufficient accuracy, but the subsequent callup is performed on a carrier of another base station of sufficient accuracy. This signal can have a better signal-to-noise ratio, and can deliver a higher frequency accuracy as well as a shorter synchronization time. For example, it is possible in the case of the UMTS method to make use of each of the possible channels of the UMTS network for autocalibration. It is thereby possible to synchronize with an arbitrary channel of a specific base station of an operating company. The subsequent callup can then take place on an arbitrary channel, including another one with another operating company. It is likewise possible to carry out the synchronization and the callup with the same base station when the latter is the most suitable one at the time.

[0011] Furthermore, in the case of an advantageous development of the invention, a single multimode mobile part can be used for synchronizing the mobile communications device with mutually synchronized base stations of different standards. This leads to a synchronization that is simple, fast and versatile. Moreover, many different standards can thereby be interconnected.

[0012] Furthermore, it is advantageous when the calibration additionally takes place over the temperature range, for which purpose a temperature measurement is combined with the frequency search algorithm. Such a calibration over the temperature would be particularly time-consuming and cost-intensive to produce. Owing to the continuous variation of the surroundings of the mobile part, a constant and optimum adaptation to the current circumstances can be performed by such a calibration during the frequency acquisition.

[0013] The invention is described in more detail below with the aid of a preferred exemplary embodiment, which is explained with the aid of the figures listed below, and in which:

[0014]FIG. 1 shows a systematic illustration of a method according to the invention, and

[0015]FIG. 2 shows a simulation result from the method illustrated in FIG. 1.

[0016] The method illustrated in FIG. 1 is explained by way of example with the aid of an application in the UMTS Standard. Synchronization to an accuracy of +/−3 ppm with regard to the carrier frequency used is required in UMTS, in order to be able to undertake subsequent temporal synchronization. An RF oscillator of a production series has on average a component scatter of typically +/−25 ppm in the natural frequency. This is then reduced to a natural frequency scatter of +/−3 ppm by the autofrequency calibration described below. A mobile part 1 of a mobile communications device receives a signal 4, transmitted by a base station (not illustrated) in a receiver 5. The transmitted signal 4 is from the baseband of the base station, which is arranged around approximately 2.1 GHz. The received signal 4 is demodulated using known means, and the signal 6 thus demodulated is compared in an evaluation and synchronization program 3 by means of a correlator with the pSCH sequence, which is already known in advance and uniform throughout the network. In this case, pSCH stands for Primary Synchronization Channel in accordance with the UMTS Standard.

[0017]FIG. 2 shows the output 12 of the correlator in the baseband, the frequencies illustrated being reduced by the center carrier frequency of the base station. A correlation peak 13 is formed at a frequency of 10 MHz. The mobile station in which channel raster the base station is transmitting is known a priori. At the base station end, the carrier frequency can [lacuna] by support of radio signals such as, for example, DCF77 or GPS with an accuracy that is still below that of the specification of the UTMS Standard of 0.05 ppm. The deviation of the RF oscillator 2 of the mobile station 1 can be determined from the comparison of the known channel frequency of the base station with the measured correlation peak 13 of the output 12 of FIG. 2. The data for the autocalibration are obtained therefrom. The production-induced scatters in the natural oscillator frequency of the RF oscillator 2 of +/−25 ppm can thereby be compensated to the extent that it is possible to assume a maximum deviation of +/−3 ppm for future synchronization operations. Future synchronization processes are substantially accelerated thereby. This saves time and reduces the energy required for calibration, something which reduces the power consumption and thereby permits longer standby times of the mobile part.

[0018] Digital settings are obtained by means of the step, illustrated in FIG. 1, of the comparison in the evaluation and synchronization program 3. During a synchronization, the settings are passed on via a switch 7 to a memory 8 in which these are stored. The digital settings are also passed on to a digital-to-analog converter 9. The latter passes on the analog signal, which corresponds to the digital settings, to the voltage-controlled RF oscillator 2. On the one hand, the digital settings are passed on to further modules 11, and on the other hand they are passed on as new reference frequency f_(ref) to the receiver 5 via a line 10.

[0019] On the occasion of the next calibration, this new reference frequency f_(ref) serves as reference signal for the signal 4 then received, which is transmitted by the base station. A new calibration is undertaken using the steps set forth above after the demodulation of the demodulated received signal 6, passed on by the receiver 5, in relation to the evaluation and synchronization program 3. In which case, the synchronization can be performed more quickly, since the current reference frequency f_(ref) is affected by a smaller scattering value. 

1. A method for calibrating the frequency of an RF oscillator (2) in a mobile part (1) of a mobile communications device when the mobile part (1) of the mobile communications device is first switched on, having the following steps: a) sampling a modulated carrier that is transmitted by a base station and has a prescribed frequency accuracy, b) determining the frequency error between the frequency of the RF oscillator (2) in the mobile part (1) of the mobile communications device and a known channel frequency of the base station, c) synchronization of the mobile communications device with the modulated carrier, characterized in that in step a) the scanning of the modulated carrier is performed by means of a frequency search algorithm in the case of which the modulated carrier is demodulated, the signal (6) thus demodulated is compared in an evaluation and synchronization program (3) by means of a correlator with a sequence known in advance, and a correlation peak (13) in the output (12) of the correlator is determined, in step b) the frequency error is determined by comparing the measured correlation peak (13) of the output (12) with a channel raster, known in advance to the mobile communications device, of the base station, and the frequency, corrected by the frequency error, for the RF oscillator (2) is adopted as reference frequency (f_(ref)) for further calibration operations.
 2. The method as claimed in claim 1, characterized in that the method is carried out under the UMTS Standard, and the sequence is the pSCH sequence uniform throughout the network.
 3. The method as claimed in claim 1 or 2, characterized in that this method is carried out repeatedly, in particular with each xth callup, starting in each case from the reference frequency (f_(ref)) last determined.
 4. The method as claimed in one of the preceding claims, characterized in that use is made during the first synchronization of a transmitted signal (4) of an arbitrary base station, but the subsequent callup is performed on a carrier of another base station.
 5. The method as claimed in claim 4, characterized in that a single multimode mobile part is used for synchronizing the mobile communications device with mutually synchronized base stations of different standards.
 6. Method according to one of the preceding claims, characterized in that a calibration is additionally performed over the temperature range by means of a temperature measurement. 